A nitride semiconductor material such as gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (GaAlN) has a large forbidden band width, and interband transition is a direct transition. Therefore, development of a short wavelength light emitting element for use in a visible light and an ultraviolet light is progressed.
In a nitride semiconductor substrate, a vapor pressure of nitrogen is extremely high, and therefore a bulky crystal growth from a melt is extremely difficult. For this reason, as a mainly used method for manufacturing the nitride semiconductor substrate, as shown in FIG. 4, a nitride semiconductor layer 22 is hetero-epitaxially grown on a heterogeneous substrate 21 different from a nitride semiconductor such as a sapphire substrate, a silicon substrate, or a gallium arsenide substrate by mainly using a vapor growth method (FIG. 4(a)), and thereafter the heterogeneous substrate 21 is removed by using a method such as separating, polishing or etching (FIG. 4(b)), and front/back surfaces of the nitride semiconductor layer 22 formed on the heterogeneous substrate 21 are subjected to polishing, etc, to thereby obtain a so-called “freestanding substrate” (FIG. 4(c)). Note that arrows in FIG. 4 show main plane directions of crystal, such as the C-axes in sapphire and GaN.
Thus, as a specific method of manufacturing the freestanding substrate, for example, a method described in patent document 1 is known.
Regarding a growth method of a nitride semiconductor layer, as described above, the nitride semiconductor layer is hetero-epitaxially grown on the heterogeneous substrate different from the nitride semiconductor. Therefore, a large crystal lattice distortion occurs at an initial time of growth due to a large difference in lattice constant, resulting in generating a dislocation density of 109 to 1010 cm−2. Such crystal defects cause a remarkable lowering of reliability of a light emitting device such as an LD (Laser diode) and an LED (Light emitted diode). Therefore, the dislocation density must be reduced.
In recent years, as a method of reducing density of such a defect, a growth technique such as ELO (epitaxial lateral overgrowth), FIELO (facet initiated epitaxial lateral overgrowth), and Pendeo epitaxy growth, are reported. By these growth techniques, propagation of dislocation from a base crystal is prevented, by forming a mask patterned with SiO2, etc, on GaN grown on a substrate such as sapphire, then making a GaN crystal further selectively grown from a window part of the mask, and covering the surface of the mask with GaN by its lateral growth. Owing to such a development of the growth technique, the dislocation density in GaN can be tremendously reduced to about 107 cm−2.
Further, there are proposed various methods in which a GaN layer, with dislocation density reduced, is epitaxially grown thick on the heterogeneous substrate such as the sapphire substrate, which is then separated from the base after growth, and the GaN layer is used as a freestanding GaN substrate. For example, there is proposed a method such that the GaN layer is formed on the sapphire substrate by using the aforementioned ELO technique, and thereafter the sapphire substrate is removed by etching, etc, to thereby obtain a GaN freestanding substrate.
In addition, VAS (Void-assisted Separation: for example, see non-patent document 1) and DEEP (Dislocation Elimination by the Epi-growth with inverted-Pyramidal pits: for example, see non-patent document 2), etc, are disclosed. VAS makes it possible to realize both separation and low dislocation of a GaN layer simultaneously, by growing the GaN layer via a void layer and a TiN thin film having a net structure on a base substrate such as a sapphire substrate, etc. Also, by this DEEP, GaN is grown on the GaAs substrate that can be removed by etching, etc, by using a patterned mask made of SiN, etc, then a plurality of pits surrounded by facets are purposely formed on a bottom of the pits, and dislocations are accumulated on the bottom of the pits, to thereby obtain a low dislocation in other region.    (Patent Document 1)    Japanese Patent Laid Open Publication No. 2002-57119    (Non-patent document 1)    Y. Oshima et. al., Jpn. J. Appl. Phys., Vol. 42 (2003), pp. L1-L3    (Non-Patent Document 2)    K. Motoki et. al., Jpn. J. Appl. Phys., Vol. 40 (2001), pp. L140-L143